The tab to retrofit Phoenix Lake is rising as Ross Valley flood control cost estimates soar.

Marin County officials say that a complicated five-part plan to convert the reservoir above Ross into a flood-control basin, once estimated to cost $15.3 million, may instead balloon to $19.7 million.

At issue is creation of an earthquake-safe reservoir that can collect storm runoff while also storing drinking water and serving as a recreation and habitat enhancement facility.

The $4.4 million cost increase came after analysis of water quality in the lake and Ross Creek indicated additional expense was needed to cover "the most conservative mitigation measures that could be required by regulatory agencies," according to Tracy J. Clay, a county principal engineer. These include complex, costly improvements such as ensuring the right mix of cool, clean, aerated water flows into the creek, home of steelhead trout.

A state water resources grant covers up to $7.6 million, or half of the initial $15.3 million estimate provided in an initial county grant application, but Ross Valley Flood Control District Zone 9 is on the hook for the rest, including all of the cost increase.

"After the grant award during the work plan development phase, information was received regarding existing water quality conditions ... that could require additional mitigation measures," Clay said in a report to county supervisors, who are in charge as county flood control directors.

The result is that the Ross Valley district "is obligated to provide a funding match from eligible sources estimated to be $12,030,150," she added, noting much of the tab will be covered by the district's flood fee. But if technical and regulatory work indicates the project cannot be completed, "the district will be responsible for 100 percent of the cost incurred and will have to return all reimbursed funds to the state," Clay reported.

County officials recently expressed concern about escalating cost estimates, but approved proceeding with an initial $4.2 million phase, half of which will be paid for by the state grant, half by flood fee revenue.

"The estimate of the total project cost has gone up," noted Supervisor Katie Rice, who has championed the project, a legacy of her former boss, the late Supervisor Hal Brown. "I'm assuming we have a high confidence level (the project) can be performed," added Supervisor Kate Sears, because if it isn't, local residents pick up all costs.

Clay said the Phoenix effort will move ahead in five phases or projects, with each project having "one or more physical elements, which is a new facility or improvement to an existing lake facility."

While the new water quality measures have increased cost estimates, other phases of the project could cost less than expected, she said.

A cost breakdown includes $13.8 million for "flood damage reduction," including retention basin improvements, modification of the dam, spillway and reservoir, seismic strengthening and landslide stabilization, lake excavation, erosion control, and a motorized drainage pipeline network. "Water supply" improvements estimated at $825,000 include raising the spillway six feet to add 120 acre-feet of storage capacity, as well as a piping system transferring water to Bon Tempe Lake. "Water quality" improvements are pegged at $425,000 for devices to oxygenate and circulate water.

In addition, the Phoenix project includes a $2.8 million "ecosystem restoration" component aimed at cooling, cleaning and aerating water released to Ross Creek, a program requiring complex plumbing, motor-controlled gates releasing precise flows, as well as a drainage and "seepage control" system involving synthetic liners.

The program also involves a $1.8 million "recreation and public access" project to improve roads, trails, culverts, reduce erosion and provide for public safety including signs, benches, information kiosks and bathrooms.

The project launches the county's overall $165 million plan to replumb the Ross Valley, a region devastated by flooding from periodic storms for more than a century.

The Phoenix Lake retrofit, which could take up to seven years to complete, is part of $135 million in planned improvements including four detention basins in the Ross Valley that can hold runoff, releasing it gradually to curb flooding during heavy storms. Other projects call for runoff basins at Lefty Gomez Field in Fairfax, Loma Alta open space in Fairfax and Memorial Park in San Anselmo. Transforming Memorial Park would cost about $6.5 million, and will probably proceed before the $6 million Lefty Gomez or $2 million Loma Alta projects.

In addition, federal plans costing up to $30 million call for dredging the lower Corte Madera Creek channel in Kentfield, Greenbrae, and Larkspur, a project that could take place by 2017.

Officials say more than a third of the Ross Valley flood basin program will be paid for with flood fee revenue that was approved in an unusual election that required voters to sign ballots. The hotly-contested election split the community but was upheld by the state Supreme Court. The fee averages roughly $125 a year.